LEDs are elements that emit light when an electrical current flows in a forward direction across a p-n junction of a semiconductor, manufactured using group III-V semiconductor crystals such as GaAs and GaN. In recent years, advances in semiconductor epitaxial growth techniques and light-emitting element processing techniques have led to development of LEDs with excellent conversion efficiency, and their broad usage in various fields.
LEDs are composed of a photoactive layer sandwiched between a p-type layer and n-type layer formed by epitaxial growth of group III-V semiconductor crystals on a single-crystal growth substrate. In general, LED light-emitting elements are formed by epitaxially growing crystals of a group III-V semiconductor such as GaN on a growth substrate of single-crystal sapphire or the like, then forming electrodes etc. (Patent Document 1).
When epitaxially growing group III-V semiconductor crystals on a single-crystal growth substrate, it is difficult to grow good single crystals due to the difference in lattice constant between the single-crystal growth substrate and the group III-V semiconductor crystals. For this reason, a method of forming a buffer layer of AN or the like at a low temperature on the sapphire substrate, then epitaxially growing GaN thereon has been proposed (Patent Document 2). However, even when using such a technique, differences in the coefficient of linear thermal expansion between the sapphire substrate and the GaN can lead to warping of the substrate after epitaxial growth, and in the worst case, the substrate may crack. For this reason, a substrate material with a coefficient of linear thermal expansion close to that of group III-V semiconductor crystals is sought.
On the other hand, single-crystal growth substrates such as single-crystal sapphire substrates have the problem of poor thermal conductivity. In the case of single-crystal sapphire, the thermal conductivity is about 40 W/mK, which is insufficient to adequately dissipate the heat generated by a group III-V semiconductor such as GaN. In high-power LEDs using large currents, the device temperature can rise and reduce light-emitting efficiency and device life. For this reason, a method has been proposed of epitaxially growing a group III-V semiconductor crystal on a single-crystal growth substrate, then bonding a high thermal conductivity substrate via a metal layer, and subsequently removing the single-crystal growth substrate (Patent Document 3). As high thermal conductivity substrates for this purpose, materials excelling in thermal conductivity such as copper have been considered, but they are not satisfactory for use with high-power LEDs as they differ significantly from group III-V semiconductor crystals in coefficient of thermal expansion.    Patent Document 1: JP 2005-117006 A    Patent Document 2: JP H5-73252 B    Patent Document 3: JP 2006-128710 A